Injecting small amounts of a gaseous material into a liquid medium is a well known method for altering the physical characteristics of the liquid medium. Three applications where this technique is utilized are: (1) protection of underwater objects as described in U.S. Pat. No. 5,992,104 to Hudak; (2) drag reduction as described in U.S. Pat. No. 5,117,882 to Stanford), U.S. Pat. No. 5,613,456 to Kuklinski, U.S. Pat. No. 6,356,816 to Katz, and U.S. Pat. No. 6,982,926 to Tenghamn; and (3) modifying transmitted and received seismic signals as described by U.S. Pat. No. 2,954,090 to Burg, U.S. Pat. No. 4,618, 024 and U.S Pat. No. 4,632,213 to Domenico, U.S. Pat. No. 4,625,302 to Clark, U.S. Pat. No. 5,959,938 to Behrens and U.S. Pat. No. 6,606,278 to Lee. The last publication is of particular interest because it provides a means to simplify the recorded seismic data and to alter the inherent signature of a marine energy source.
Injecting small amounts, typically less than a few percent by volume, of gaseous material into a liquid medium dramatically increases the effective compressibility with a corresponding reduction in the acoustic velocity of the fluid-gas mixture. As taught by Domenico and others, when acoustic waves impinge on and pass through a liquid medium containing gaseous bubbles, a complex system of energy reflection, refraction and attenuation is created. The response of the seismic wave to a gaseous mixture is frequency dependent and is principally a function of the percentage volume taken up by the gaseous material, the thickness of the gaseous zone and the size of the gaseous bubbles. These references teach creating bubble layers that shield the air-water surface from direct and indirect seismic source energy, thus preventing surface reflections that would contribute noise at receivers in a marine seismic survey, or the bubble layers alter the surface reflection in a geophysically significant manner so as to improve the signal-to-noise ratio seen at the receivers.
To create inhomogeneous regions in the water column, Burg, Domenico, Clark and Behrens utilized discrete holes or nozzles with a single or a few hole diameters. Depending on the orientation, air-bubble volume percent or intended use, the shape of the inhomogeneous region has been referred to as a bubble curtain or an acoustic blanket or an acoustic lens. In practice, the shapes are used to reflect, refract and/or attenuate seismic energy generated by marine energy sources. Lee also builds inhomogeneous regions in the water column; but he creates the inhomogeneous regions using microbubbles created with porous wall tubing.
For drag reduction, Tenghamn makes use of discrete holes or perforations. Stanford, Kuklinski and Katz utilize microbubbles to reduce drag caused at the turbulent boundary layer. In addition Katz, references the use of slider plates to achieve variable bubble sizes.
For both the seismic signal modification application and the drag reduction application, the range of bubble sizes is limited to a fairly narrow range by the bubble production mechanism. Bubble generation mechanisms such as metal fibers structures, sintered powder metal, ceramic stones and porous plastics and rubber structures have effective pore sizes less than 200 to 400 microns. These types of mechanical systems are used to generate what are generally termed microbubbles. Discrete holes, typically greater than 200 microns in diameter, create larger bubbles. For both of these bubble generation techniques, the size of the bubbles can be altered by the hole spacing, fluid flow at the bubble generation site, the effective surface tension during bubble generation and the differential pressure between the gaseous region and the fluid region. For the seismic application, it is advantageous to have a broad range of bubble sizes so as to affect a broad frequency range. A diversity of bubble sizes also increases the range of bubble rise rates, which adds to the complexity of the bubble field.
Other related references include:                Domenico, “Acoustic wave propagation in air-bubble curtains in water—Part I: History and theory,” Geophysics 47, 345-353 (1982);        Domenico, “Acoustic wave propagation in air-bubble curtains in water—Part II: field Experiment: Geophysics 47, 354-375 (1982);        Sixma and Stubbs, “Air Bubble Screen Noise Suppression Test in Lake Maracaibo,” Congresso Venezolano de Geofisica (1996);        Ross et al., “Mitigating seismic noise with an acoustic blanket—the promise and the challenge,” The Leading Edge 24, 303-313 (2005).        